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Iotation occurs when a labial (/m/, /b/), dental (/n/, /s/, /l/) or velar (/k/, /ɡ/, /x/) consonant comes into contact with a iotified vowel, i.e. one preceded by a palatal glide /j/. As result, the consonant becomes partially or completely palatalized.[1] In many Slavic languages, iotated consonants are called "soft" and the process of iotation is called "softening".

Iotation can result in a partial palatalization so the centre of the tongue is raised during and after the articulation of the consonant. There can also be a complete sound change to a palatal or alveolo-palatal consonant. This table summarizes the typical outcomes in the modern Slavic languages:

According to most scholars, the period of iotation started approximately in the 5th century, in the era of Proto-Slavic, and it lasted for several centuries, probably into the late Common Slavic dialect differentiation. Here are examples from the early stage:[1]

In Slavic languages, iotified vowels are preceded by a palatal approximant/j/ before a vowel, at the beginning of a word, or between two vowels in the middle of a word, creating a diphthongoid, a partial diphthong.[2] In the Greek alphabet, the consonant is represented by iota (ι). For example, the Englishapple is cognate to Russianяблоко (jabloko): both come from Proto-Indo-European stem *ābol-. As a result of the phenomenon, no native Slavic root starts with an [e] or an [a] but only with a [je] and [ja]; although other vowels are possible.

As it was invented for the writing of Slavic languages, the original Cyrillic alphabet has relatively complex ways for representing iotation by devoting an entire class of letters to deal with the issue. There are letters which represent iotified vowels; the same letters also palatalize preceding consonants (with or without self-iotation), which is why iotation and palatalization are often mixed up. There are also two special letters (soft signЬ and hard signЪ) that also induce iotation; in addition, Ь palatalizes preceding consonant, allowing combinations of both palatalized (soft) and plain (hard) consonants with [j]. Originally, these letters produced short vowels [i] and [u]. The exact use depends on the language.

The adjective for a phone which undergoes iotation is iotated. The adjective for a letter formed as a ligature of the Early Cyrillic I (І) and another letter, used to represent iotation, is iotified.[citation needed]. The use of a iotified letter does not necessarily denote iotation. Even a iotified letter following a consonant letter is not iotated in most orthographies, but iotified letters imply iotated pronunciation after vowels, soft and hard signs as well as in isolation

Iotated consonants occur as result of iotation. They are represented in IPA with superscript j after it and in X-SAMPA with apostrophe after it so the pronunciation of iotated n could be represented as [nʲ] or [n'].